Replacing a Roof

Dear NH,
At the risk of a book length reply, I am going to redo my roof this summer, and am wondering if it is better to remove the old asphalt shingles, or (as many people have told me) just lay the new shingles on top of the old. There is only one layer on the roof now.
JG from Exeter, NH

That is actually a great and very straight-forward question, with an equally great and straight-forward answer. You can generally lay a new roof over the old without any problems as long as:

  1. You only have one layer of asphalt shingles on the roof. Most roof frames can support two layers of asphalt shingles, rarely three. Depends on the framing, of course.
  2. The shingles lay fairly flat. The new shingles will take on the shape of the old, so if the old shingles are very curled, twisted, or buckled, the new roof will eventually look the same AND the life span of the new shingles may be decreased. This may also void the manufacturer's shingle warranty.
  3. There are no existing leaks from undetermined sources. Just putting a new roof over an old roof will not necessarily fix underlying problems, such as with flashing or roof vents. These leaks will most likely recur unless they are properly repaired prior to installation of the new roof.
  4. You were satisfied with the performance of the old roof... e.g.. no occasional leaks during rainstorms, etc. This refers back to item (3)... new roof over old roof may not guarantee elimination of leaks.

Ideally, your or your contractor should repair, reseat, or (ideally) replace all flashing around chimneys, valleys, vents, etc. He should also cut off or replace any curling shingles so the new ones lay flat. If you have had ice dam leakage, he should also remove the bottom few feet of old shingles and install an ice and water shield right against the roof sheathing plywood. You can then be assure that your roof will have a long and healthy life!


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